This invention relates to plasma X-ray sources of the Z-pinch type and, more particularly, to plasma X-ray sources that utilize a surface discharge to initiate a plasma discharge at relatively low gas pressures.
A Z-pinch plasma X-ray source that utilizes the collapse of a precisely controlled low density plasma shell to produce intense pulses of soft X-rays or extreme ultraviolet radiation is disclosed in U.S. Pat. No. 5,504,795 issued Apr. 2, 1996 to McGeoch. The X-ray source includes a chamber defining a pinch region having a central axis, an RF electrode disposed around the pinch region for preionizing the gas in the pinch region to form a plasma shell that is symmetrical around the central axis in response to application of RF energy to the RF electrode, and a pinch anode and a pinch cathode disposed at opposite ends of the pinch region. An X-radiating gas is introduced into the chamber at a typical pressure level between 0.1 torr and 10 torr. The pinch anode and the pinch cathode produce a current through the plasma shell in an axial direction and produce an azimuthal magnetic field in the pinch region in response to application of a high energy electrical pulse to the pinch anode and the pinch cathode. The azimuthal magnetic field causes the plasma shell to collapse to the central axis and to generate X-rays.
While the disclosed X-ray source is very effective for pinch plasmas driven by upwards of 100 joules (J) of stored energy, the requirement for use as a source in scanning ring field cameras in the process of extreme ultraviolet lithography is for repetition frequencies in excess of 1 kilohertz at lesser stored energy. In this application, it may be desirable for stored energies much less than 100 J to be used, with a preferred range between 10 and 100 J for the Z-pinch X-ray source. With such low applied energies, a proportionately lower initial gas density is required in order to reach the same plasma temperature and to radiate in the extreme ultraviolet bands of interest. The reduced gas density, however, increases the difficulty of ignition of the pinch discharge, because the electron mean free path is comparable to the dimensions of the pinch chamber. Such conditions involve the density regime on the lower side of the so-called xe2x80x9cPaschen minimumxe2x80x9d in the plot of gas breakdown voltage as a function of the product of gas density times the characteristic dimension of the apparatus, where rapidly increasing voltage is required to break the gas down in order to carry a high current discharge.
In this circumstance, it is found that radio frequency preionization is less effective for two reasons. Because of electron losses on the walls of the chamber, there is an increasing probability that the preionizer discharge fails to ignite before the main current pulse is applied. Also, the radio frequency discharge becomes more diffuse, extending almost uniformly throughout the cylindrical pinch chamber, and is incapable of reliably initiating the main pinch discharge near the chamber walls, as can be achieved at higher gas density.
Accordingly, there is a need for improved preionization techniques in Z-pinch plasma X-ray sources which operate at low gas densities.
According to a first aspect of the invention, a Z-pinch plasma X-ray source is provided. The plasma X-ray source comprises a chamber containing a gas at a prescribed pressure, the chamber comprising an insulating wall and defining a pinch region having a central axis, a pinch anode disposed at one end of the pinch region, a conductive shell surrounding the insulating wall and electrically connected to the pinch anode, and a pinch cathode disposed at the opposite end of the pinch region. The plasma X-ray source further comprises a first conductor defining an edge in close proximity to or contacting an inside surface of the insulating wall, and a second conductor disposed around an outside surface of the insulating wall, wherein a surface discharge is produced on the inside surface of the insulating wall in response to application of a voltage to the first and second conductors. The surface discharge causes the gas to ionize and to form a plasma shell near the inside surface of the insulating wall. The pinch anode and the pinch cathode produce a current through the plasma shell in an axial direction and produce an azimuthal magnetic field in the pinch region in response to an application of a high energy electrical pulse to the pinch anode and the pinch cathode. The azimuthal magnetic field causes the plasma shell to collapse to the central axis and to generate X-rays.
In a first embodiment, the first conductor is coupled between the cathode and the insulating wall. In a second embodiment, the first conductor comprises the cathode, and the cathode is tapered toward the insulating wall to define the edge. In either case, the surface discharge is initiated at the edge of the first conductor and propagates along the inside surface of the insulating wall toward the pinch anode. In a third embodiment, the second conductor comprises the conductive shell that surrounds the insulating wall. In this embodiment, the surface discharge is initiated upon application of the high energy electrical pulse to the pinch anode and the pinch cathode.
In a fourth embodiment, the second conductor comprises a preionizer control electrode positioned between the conductive shell and the insulating wall. The preionizer control electrode is coupled to a preionizer voltage source which, for example, may be a radio frequency source. In this embodiment, the preionizer voltage may be applied to the preionizer control electrode prior to application of the high energy pulse to the pinch anode and the pinch cathode. In the fourth embodiment, the preionizer control electrode may comprise a single element or a plurality of elements having separate voltages applied thereto.
In one embodiment, the gas utilized in the Z-pinch chamber may comprise xenon for the generation of extreme ultraviolet radiation in a band between 100 angstroms and 150 angstroms. In another embodiment, the gas may comprise lithium for the generation of the doubly ionized lithium resonance line at 135 angstroms. A carrier gas may be utilized to deliver and remove lithium vapor.
According to another aspect of the invention, a Z-pinch plasma X-ray system comprises a plasma X-ray source as described above, a gas supply system coupled to the chamber of the plasma X-ray source and a drive circuit connected to the pinch anode and the pinch cathode for applying the high energy electrical pulse to the pinch anode and the pinch cathode. In one embodiment, the drive circuit comprises a solid state switched pulse generator with magnetic pulse compression.
The gas supply system may comprise a vacuum pump coupled to the pinch region for recompression of exhaust gas pumped from the pinch region and for recirculating the gas to the pinch region. The gas supply system may further comprise a filter module for filtration and purification of the gas exhausted from the pinch region prior to its return to the pinch region.
The plasma X-ray system may further comprise a barrier plate located on the axis outside the pinch region. The barrier plate has a multiplicity of aligned holes for passing soft X-rays or extreme ultraviolet radiation, while impeding the flow of gas from the pinch region.
According to a further aspect of the invention, a method is provided for generating soft X-rays or extreme ultraviolet radiation in a Z-pinch plasma X-ray source comprising a Z-pinch chamber containing a gas at a prescribed pressure, a chamber comprising an insulating all and defining a pinch region having a central axis, a pinch anode disposed at one end of the pinch region and a pinch cathode disposed at an opposite end of the pinch region. The method comprises the steps of producing on an inside surface of the insulating wall a surface discharge that causes the gas to ionize and to form a plasma shell near the insulating wall, and applying a high energy electrical pulse to the pinch anode and the pinch cathode to produce a current through the plasma shell in an axial direction and to produce an azimuthal magnetic field in the pinch region. The azimuthal magnetic field causes the plasma shell to collapse to the central axis and to generate X-rays.